1. Field of the Invention
This invention relates to a process for the recovery in the form of tetravalent fluoride of uranium from an aqueous phosphate-bearing solution
2. Discussion of the Background
Phosphate-bearing solutions and in particular phosphoric acid which are produced by sulphuric attack on phosphate-bearing ores contain an amount of uranium which is generally from 0.040 to 0.200 g/l, depending on the origin of the ores.
Because of the high level of interest in uranium in the nuclear industry many processes have been developed for recovering that metal from such solutions.
Some of the processes involved consist of extracting uranium in hexavalent form by using organophosphorus solvents such as mixtures of di-2-ethylhexylphosphoric acid and trioctylphosphine oxide. This organophosphorous mixture is commonly referred to as di-2-EHPA+TOPO, and is described for example in U.S. Pat. No. 3,711,951.
Other processes are based on extracting uranium in tetravalent form by using other organophosphorus solvents such as for example mixtures of mono- and dioctylphenylphosphoric acids. These processes are referred to as O.P.A.P. processes. When alkylpyrophosphoric acids such as octylpyrophosphoric acid are used, the process is referred to as the O.P.P.A. process. The O.P.A.P. process is disclosed in U.S. Pat. No. 3,835,214 and Belgian Pat. No. 870,346. The O.P.P.A. process is disclosed in U.S. Pat. No. 2,866,680.
When uranium is extracted in tetravalent form, a uranium-charged organophosphorus solvent is generally contacted with an aqueous solution hydrofluoric acid to extract the uranium via the formation of a suspension of UF.sub.4 which can be recovered from the aqueous solution after separation of the solvent.
In order to achieve a suitable rate of reextraction, it is generally necessary to use a least a 15% by weight HF solution. However, the organophosphorus solvents degrade in the presence of such solutions. The degradation becomes increasingly severe as the concentration of HF is increased and contact time with the solvent is increased. In a process for the continuous extraction of uranium, it is therefore necessary to use a solvent composition which compensates for loss due to degradation. Thus, with the O.P.P.A. process it is the general practice to contiuously add at least 10 g of octylpyrophosphoric acid per litre of solution passing into the extraction battery.
There is thus a strongly felt need for an efficient process for the extraction of uranium in tetravalent form. Such a process would have to minimize costly material loss (solvent loss) during its operation as well as provide good yields of uranium.